Encapsulation is critical to the lifetime of solar cells, especially thin film solar cells. Commonly used encapsulation methods (Glass/TF-Cell/PVB/Glass) or Glass/TF-cell/EVA/Tedlar/Al) show a considerable amount of moisture and/or oxygen permeation, mainly from the edges of the solar cell through the EVA/PVB lamination material. The amount of diffusing species depends on the PVB or EVA area (thickness of the encapsulation material film times perimeter of the module) directly exposed to the atmosphere. Consequently a thick PVB or EVA layer results in a high permeation probability, raising the amount on humidity and/or oxygen within the encapsulated area. The gases permeated through the encapsulation material may be harmful to some TF cell materials. In particular boron-doped ZnO, used as a transparent conductive layer, is moisture sensitive and may be severely lifetime limited in case standard encapsulation is used.
As a consequence, module manufacturers are using stable thin film materials like SnO2. These materials are the best choice regarding cell stability. However, this may not be in line with the best choice for overall cell performance. For example the moisture sensitive ZnO transparent electrode material increases the cell performance, especially the light trapping behavior (see J. Mueller in Solar Energy 77 (2004) p. 917 or J. Meier et al at Orlando Solar Energy Conference 2005 proceedings (2005)). Due to the limiting isolation qualities of the encapsulation material, this superior transparent electrode material is not used, to avoid degradation of the cell when exposed to humidity.
FIGS. 1a and 1b show currently used encapsulation schemes for a TF cell in a solar collection module 100. The TF cell 20 is encapsulated between a transparent substrate 10 at the front and an encapsulation material 30 (EVA or PVB) and a back protection 40 at the back. This back protection 40 may be glass 41 or a polymeric material 42. In case a polymeric foil 42 is used, an additional metal layer 43 attached to the polymeric foil is needed to limit the diffusion through the polymeric foil. Currently, rather thick polymeric foils or layers (up to several mm) are used and the metal film is directly laminated on the polymeric foil. The metal film and the polymeric foil are provided having flush edges, and need to be attached to the glass substrate. Attachment to the substrate can be accomplished by lamination or a gluing process. Tedlar is the material of choice for the polymeric foil mentioned above since it is very stable against environmental conditions. However other foil materials may be used.
A more sophisticated approach in encapsulating a thin film solar cell 20 with non-glass alternatives is the use of a back protection 40 comprising a metal sealing foil 60 as shown in FIG. 2. The mostly polymer foil 50 is used as a dielectric material and a mechanical protection while the metal foil 60 is used as a diffusion barrier for environmental gases like oxygen or water vapor. For industrial use the metal and the polymer foil 60 and 50 are attached to each other mostly by gluing, with the metal and polymer foil having flush edges. Laminating this specific back protection 40 to the glass substrate 10, as shown in FIG. 2, results in a solar module 100 with reduced moisture permeation. However this encapsulation technique has the major drawback of arcing problems.
The arcing results from leak current which is flowing from the metal edge of the multilayer foil (edge of foil 50/60) to the grounded metal frame 90 or any surrounding ground potential. Modern, transformer-less DC/AC converters switch the polarity of the modules to convert to AC voltage more efficiently. The TF-cell 20 and the back protection 40 with the metal layer 60 represent a capacitor in respect to this switching. As a consequence, the voltage on the metal edge follows the voltage of the TF-cell 20. Since the metal 60 foil is charged now, arcing to ground potential (for example the module frame) is possible and will most likely occur during humid conditions. This arcing not only damages the encapsulation foil but also causes severe problems to the DC/AC converters.
The currently available encapsulation scheme limits the module design to materials which are not moisture and/or oxygen sensitive. For example, currently, thin film module manufacturers are using SnO2 for transparent electrodes. ZnO with a high haze factor would be an alternative. However this material is more moisture sensitive. The use of this rough ZnO would also enable new cell designs. Microcrystalline p-layers are usually deposited in a reducing atmosphere which is capable of reducing SnO2 to native tin. This reaction reduces the transparency of the SnO2 front contact. As a consequence the cell performance is lowered. In contrast, ZnO does not show this behavior when exposed to reducing plasmas during player deposition, allowing the direct deposition of a microcrystalline player on the transparent electrode.
Similarly a moisture/air permeation barrier will give the flexibility for a lot of other alternative cell designs like alternative back reflectors made from ZnO/Al or NIP cells. The currently applied scheme as shown in FIG. 1 uses rather thick layer of EVA or PVB. Both materials show some moisture/air diffusion. To protect the TF cell design, the distance between the module edge 15 and the TF cell should be as big as possible. On the other hand the dead area between the edge 15 and the TF cell 20 is basically reducing the power of the module. As a consequence the overall value of the deposited glass plate is lowered by this unused area.
In general glass plate based encapsulation schemes add extra weight to the module. As long as the module area is small this is not a major concern. As soon as the glass size is exceeding the 1 m2 area the module weights increase above 10 kg depending on the glass thickness. Adding the weight of the encapsulation glass to this weight may require the usage of special mounting tools like cranes or other lifting devices for mounting the module. The increased weight of the module 100 due to the glass back plate also influences the supporting structure of the module at the end-user side.
As a consequence (FIG. 1b), instead of a glass based back side also polymeric (plastic) foil 42 based cell covers (back protection 40) are used. These schemes usually need a metal foil 43 on top of the plastic material to achieve reasonable moisture/air permeation values. In case this design is used other problems must be considered. For example modern module and solar cell technology is alternating the current in the DC/AC converter changing the current on the module frequently. These current changes charge the metal back foil 43 causing arcs between the metal foil 43 and the frame 90 holding the solar module 100. These arcs may cause the total loss of the solar module 100. As a consequence, the encapsulation must be set in a distance to the metal frame 90. So a distance between the frame 90 and the encapsulated area is provided, which increases the distance between the glass edge 15 and that of the energy collecting material (TF cell 20). Correspondingly, the fraction of TF cell area on the glass substrate 20 is reduced causing a loss in active area on the solar cell module 100.
Current back foil 60,50 approaches are using a combination of foil-metal-Tedlar® for encapsulating thin film cells. The final Tedlar® foil is mostly used for weather and environmental protection and rather highly priced. Alternative final encapsulation materials would be highly appreciated by industry.
Nevertheless the main problems in solar cell encapsulations arise on the edge 15 of the module 100. There the encapsulation-substrate interface is highly vulnerable to adhesion problems. Moisture/air creeping through capillary effects on this interface may show up destroying the performance of the cell. Such capillary effects may be enhanced by insufficient cleaning of the interface to be bonded or by adhesion problems due to improper cleaning or handling of the glass, the back foil or the glue.